feat(web): notes render as markdown — markdown-it+DOMPurify pipeline, rich paste

Objective 6 (web half). Storage/sync stay raw markdown (KOReader shows
literal source, accepted). Notes-grade syntax: headings, emphasis/
strikethrough, code, links, lists, blockquotes, GFM tables, reference
links, footnotes (markdown-it-footnote); images/raw HTML/math excluded
(html:false posture + FORBID img/style/form + default safe-scheme URI
check). Display sites: annotations-drawer note rows and highlight note
lines render pre-sanitized HTML (x-html); render-time sanitize only.

Rich paste: both note textareas intercept paste; a text/html clipboard
flavor converts via turndown (gfm tables incl. a headingless-table rule
emitting pipe syntax; images blanked) and inserts at the caret; plain
pastics fall through to the default paste unchanged.

Verified live in Brave: bold/italic/strike/code/lists/quote/table/
footnote-ref/reference-link render; <script> renders inert literal
text with no alert; javascript: hrefs absent from the DOM; paste of
rich HTML lands markdown at the caret; plain pastes untouched.

web/src/shims.d.ts

@@ -0,0 +1,2 @@
+declare module "markdown-it-footnote";
+declare module "turndown-plugin-gfm";